On hand-held devices with modern color Liquid Crystal Display (LCD) and Organic Light Emitting Diode (OLED) displays, it is a fairly common User Interface (UI) technique to temporarily dim the background screen when bringing up a temporary dialog, panel, or set of selectable elements over the background screen. This feature appears as a semi-transparent “veil” that has been drawn over the background screen. The net effect is to darken the pixels in the background screen so the white pixels become gray and the gray pixels become darker gray. The active dialog, panel, or set of selectable elements are contained in a non-full-screen-sized window that contains an area of user interactivity. Along this active window's border and in the background, the underlying window that was present before the dialog was brought up is partially occluded. In general practice, the occluding along the border and in the background is often achieved with an alpha mask, that serves to darken/obscure the background window, drawing the user's attention to the foreground dialog.
Dimming the background helps users focus on the new foreground elements that have just appeared over the background, and let the user know that the background elements are not active while the new foreground elements are active.
The most common type of Electronic Paper Display (EPD) is formed using electrophoretic technology. In this technology, particles, typically titanium dioxide (titania) particles approximately one micrometer in diameter are dispersed in a hydrocarbon oil. A dark-colored dye is also added to the oil, along with surfactants and charging agents that cause the particles to take on an electric charge. This mixture is placed between two parallel, conductive plates separated by a gap of 10 to 100 micrometers. When a voltage is applied across the two plates, the particles will migrate electrophoretically to the plate bearing the opposite charge from that on the particles. When the particles are located at the front (viewing) side of the display, it appears white, because light is scattered back to the viewer by the high-index titania particles. When the particles are located at the rear side of the display, it appears dark, because the incident light is absorbed by the colored dye. If the rear electrode is divided into a number of small picture elements, pixels, in an active matrix grid, then an image can be formed by applying the appropriate voltage to each region of the display to create a pattern of reflecting and absorbing regions.
In an alternative embodiment, an electrophoretic display can use tiny microcapsules filled with electrically charged white particles suspended in a colored oil. In some versions, the underlying circuitry controls whether the white particles were at the top of the capsule (so it looked white to the viewer) or at the bottom of the capsule (so the viewer saw the color of the oil). This use of microcapsules allows the display to be used on flexible plastic sheets instead of glass.
EPD displays provide several advantages over other display types, such as LCD but exhibit poor performance and clarity under certain circumstances, notably screen updates. A screen update involves a transition on a subset of the x-y pixels in the display, denoted as ‘pixel transitions.’ In other words, a pixel transition is the change of the color value (or grayscale value) of a pixel at a given x-y address. Due to the underlying physical mechanism that EPDs use for display (pigmented substances that travel vertically through a liquid medium as a response to electrical fields in the x-y neighborhood of the pixel), unpleasant visual errors and artifacts in that neighborhood are often the by-product of a pixel transition. This is especially the case with pixel transitions that involve non-black/non-white values, either as the source or target member of the transition pair.
On an EPD screen, a typical method of creating the veil is to first drive all of the pixel to black, and then recharge them to the gray color. The reason this was done is that shifting from one shade of gray to a different, darker, shade of gray is unreliable. Specifically, the charged particles in a given pixel, do not necessarily respond as desired to a small incremental change in the charge of the electrodes controlling the pixel. Driving the pixels all the way to black or white, and then back to the desired shade of gray is much more reliable process. This process is illustrated in FIGS. 1-3.
An exemplary User Interface (“UI”) screen displayed on an EPD device 100 is illustrated in FIG. 1. As shown in FIG. 2, a dialog box 110 is displayed on the EPD device 100 with areas that are selectable by the user. At the same time as the dialog box 110 is displayed, the previous image, which now appears as a background image, is darkened by a veil 120 in order to highlight the dialog box 110 to the user. As described above, in order to reliably draw the veil 120 over the background image, the system briefly forces every pixel to a full black or full white state, as shown in FIG. 3, before the pixels that will wind up being gray are made gray as in FIG. 2. This brief transition states lasts roughly a half second on typical EPD devices. Most pixels on a page of text tend to be white, and those pixels generally get forced to black, so this brief transition state appears as a “black flash”.